The Dalecarlia Reservoir serves as a primary sedimentation basin. A portion of the water from the reservoir is treated at the nearby Dalecarlia Water Treatment Plant and distributed to municipal water mains. The remainder of the water from the reservoir flows to the Georgetown Reservoir in Georgetown. This facility serves as an additional sedimentation basin, and then the water flows through the Washington City Tunnel to the treatment facility at the McMillan Reservoir, after which it is pumped through city mains.

The system originally used a single pipe for water delivery, and did not have any water purification plants, relying instead on the reservoirs to act as settling basins. By the turn of the 20th century, however, Washington's growth and the high amount of sediment in the Potomac's water kept the reservoirs from doing their jobs well, and so the first treatment plant, a massive slow sand filter bed system, was installed at McMillan Reservoir, and was completed in 1905.[4]:Ch.4 The regular use of chlorine as a disinfectant began in 1923 at the McMillan plant. The McMillan plant was not replaced until 1985, when a rapid sand filter plant was opened adjacent to it.[1] Efforts are under way to redevelop the land the slow sand plant used, while maintaining some of the plant's sand silos for historical purposes.

In the 1920s, the Aqueduct was upgraded with the addition of a second pipe from Great Falls to Dalecarlia, along with several new reservoirs and a pumping station, A rapid sand filter plant was built at Dalecarlia Reservoir, which went online in 1927.[4]:204-205 The Dalecarlia plant is the larger of the two plants in the system, having been upgraded in the 1950s, and is the plant that serves the Virginia communities that use the Aqueduct. The Corps built an additional intake and pumping station at Little Falls in 1959.[4]:256